This invention relates to electrical contacts between printed circuit boards and electronic modules, particularly involving contact sites through land grid array (LGA) sockets.
A typical LGA interposer system comprises a printed circuit board (PCB) with electrically conductive contact pads, a module (or other printed circuit board) with a corresponding set of electrically conductive contact sites, an interposer between the module and the printed circuit board and an array of spring elements to make electrical contact between the module and the printed circuit board. Clamps are used to mechanically hold the module to the interposer and to electrically join the module contact sites through the spring elements to the printed circuit board pads.
A cooling device or heat sink is typically coupled to the module required to provide cooling of the entire electronic assembly. Many of the heat sinks have a substantial size and mass relative to the other components. This size and mass create a moment arm, causing relative movement between the module and the other components when the assembly is subjected to shock or vibration.
The spring elements used to make the electrical contact between the module sites and the PCB pads may be any one of a number of different types. Among the spring elements are metal filled elastomers, such as those sold by Tyco Inc. (formerly Thomas and Betts) as Metal Particle Interconnect Elastomers. Others are compressible wadded wires, commonly referred to as fuzz buttons shown, for example, in the following patents: U.S. Pat. Nos. 5,552,752; 5.146.453 and 5,631,446. These are small, irregularly wound and inter-twined pads or balls and are made of gold plated beryllium copper wool or gold plated molybdenum wire. Metal springs are also used. These metal springs generally are leaf springs having a number of geometries, such as C-shaped or V-shaped.
In typical LGA applications, shock and vibration can cause a variety of problems which may manifest themselves in decreased reliability and life expectancy, resulting in ongoing maintenance and repair problems. These problems can be viewed from two coordinate systems; 1) The in-plane or x-y axis, as seen when looking at an LGA interposer site, and 2) The x-z or y-z planes which are perpendicular to the board surface.
Problems Along the In-plane or x-y Axis
Typically, an interposer structure uses eight leaf springs (two per side positioned toward the corners) to center the module in an interposer housing. Using spring support on all four edges of the module provides very low (i.e. near zero) spring constant for the module during shock and vibration. As the heat sink mass increases, the natural frequency of the response decreases.
Sliding can occur between the surface of the module and the corresponding surface of the interposer. The module is held in position relative to the interposer by at least two springs on each edge of the module. The shear force between the surfaces is equal to the clamping force applied at right angles to the surfaces, multiplied by the coefficient of friction between the two surfaces.
Efforts that have been used to combat this problem include increasing the assembly clamping force. This increases the friction between the module and the interposer and tends to flatten the two components. Consequently, it increases stresses within these components, thereby leading to cracks or failures of the module and reduced product life.
As the response natural frequency of the system decreases, the alignment springs provide less module restraint during excitation. The only remaining support is the frictional contact that may occur between the module and the spring elements and/or interposer housing.
Problems Along the x-z or y-z Plane
The z-axis problem contains some additional attributes of significance. Module substrate flatness is a critical factor for module motion that is perpendicular to the printed circuit board surface. A flatness of 3 to 6 mils for a ceramic module is common in the industry today, but there is no control over whether the surface is xe2x80x98concavexe2x80x99 or xe2x80x98convexxe2x80x99. For a xe2x80x98convexxe2x80x99 module surface, the center portion of the contact array field is closer to the interposer surface than to the edge portions. There are no established standards or specifications for the flatness of the surface of the interposer, although it is common to strive for a flatness of +/xe2x88x922 mils.
When a non-flat module substrate is mated to an interposer, this center of the substrate can contact the interposer housing surface first, creating a second loading path (parallel to the spring elements). If there are approximately the same number of spring elements on either side of the contacting portions of the module and interposer, the net stiffness of the elements is again very small. When this assembly is subject to shock and vibration, the heat sink mass and movement arm tend to xe2x80x98rockxe2x80x99 the module in the interposer housing. This xe2x80x98rockingxe2x80x99 creates contact micro-motion, leading to contact wear, and electrical resistance problems. Contact motion of a small amplitude or micro-motion creates two reliability risks for an electrical contact. First is the risk of disturbing the contact xe2x80x98axe2x80x99 or asperity spot where electrical contact actually occurs. If the xe2x80x98axe2x80x99 spot is disturbed, the electrical contact must be re-established before the next pulse of a digital signal can pass through the connection. This time to re-establish would be measured in nano-seconds. Secondly, small amounts of contact motion can wear the plated precious metal layer intended to protect the contact from corrosion. If the plated layer wears through to the base material susceptible to corrosion, the electrical resistance of the contact can increase, thereby inhibiting the electrical signal from passing.
Another drawback is that there is no protocol for the assembly of the module and interposer in a manner that will provide for the two mating surfaces to be matched so that a concave portion of one body will coincide with a convex surface of the other. Thus, whenever there are non-planar contact points, micro movements in the plane or at an angle to the plane of the module and the interposer can occur.
U.S. Pat. No. 5,720,630 relates to electrical connectors that are adapted to function reliably even under conditions of extreme vibration. These serve to overcome the necessity of providing a large contact area between male and female contact sites. This decreases the degree of design flexibility for the connectors, and the weight of the connector assembly. The connectors utilize a compressible, conductive contact enabling electric signals and current to flow between male contact pins.
The present invention relates to the prevention or reduction of the contact motion during shock and vibration or other mechanical disturbance of an LGA socket, thereby substantially minimizing electrical resistance problems and mechanical failures between a printed circuit board and a module or other PCB.
One objective of the present invention is to increase the natural frequency of the module-to-LGA mounting system under a given load to accommodate more mechanical disturbance of the assembled system. As the natural frequency increases, the displacement decreases, thereby providing less module motion and increased contact life.
Another objective is to alleviate contact micro-motion and to reduce reliability problems, while at the same time supporting larger heat sink masses.
Yet another objective is to reduce rocking motion between a module, such as a ceramic module, and an interposer in situations wherein the contact surface of the module is convex with respect to the surface of the interposer.
For purposes of briefly describing the present invention, the interposer, module and circuit board are deemed to be rectangular in shape, generally flat and relatively thin in proportion to their planar surfaces. It is understood, however, that the teachings of the invention are likewise applicable to these components, even though they may have other designs, shapes, and configurations.
One solution to this motion problem in the x-y plane is to provide fixed restraints around the periphery on at least two edges of the module/interposer system. One arrangement comprises the use of two substantially rigid projections on a first edge of the interposer. At least one, and more typically two, spring members are located on an edge (i.e. edge 3) opposite of the first edge. One or two substantially rigid projections are positioned on a second edge (edge 2) that is adjacent and substantially perpendicular to the first edge to provide a second restraint. At least one, and more typically at least two, spring members are located on the edge (edge 4) opposite of the second edge, thereby creating a force toward the alignment position on the second edge.
Among the benefits of this solution along the x-y plane are:
Alignment spring rate does not essentially cancel during micro-motion;
Better absolute positional tolerance; and
During shock and vibration toward the spring members (i.e. edges 3 and 4), spring preload must be overcome before module motion is a concern.
Rocking motion along the z-axis may be caused by any convex curvature of the surface of the module facing the interposer, or partial compression of contacts leaving a gap between the adjacent surfaces of the module and the interposer insulator. This problem is solved by the use of one or more substantially rigid supports, which are provided on the interposer along the z-axis. These supports serve as stops to prevent rocking of the module relative to the interposer during shock and vibration. The force to maintain contact between the stops and the module is provided by the conventional clamping system. These rigid supports provide a support rim on the perimeter of the module that is higher than the non-flatness of the module. Preferably, the combined height of the stops is at least equal to two times the vertical distance between the perimeter of the convex surface and the top of the convex surface, whereby the stops engage the perimeter of the module without the module contacting the planar portion of the interposer.
Advantages of the z-axis solution:
A common goal for the LGA interposer system is to increase the contact compliance or contact travel under a given amount of loading to accommodate more actuation tolerance. As the contact compliance increases, the spring rate decreases, thereby allowing for greater module motion during actuation and, therefore, during shock and vibration. This z-axis support method alleviates contact micro-motion and reliability problems while supporting larger heat sink masses.
The present invention comprises an electronic assembly including a printed circuit board, an electronic module and an interposer, and a method of controlling the relative motion between the module and the interposer in such an assembly. The printed circuit board includes a plurality of electrical contact pads thereon. The module can be made of ceramic, a dielectric, plastic or other rigid material. It has a bottom surface that includes a plurality of contact sites, some of which correspond to the pads on said printed circuit board. The interposer is positioned between said printed circuit board and said bottom surface of the module and comprises an insulator and a plurality of compressible spring elements, each adapted to electrically connect one of said electrical contact pads of said printed circuit board to a respective one of said contact sites on the surface of said module. The assembly further includes means for controlling relative motion between the interposer and the module. This is achieved by the use of an interposer having two spacedly positioned supports projecting therefrom toward said module such that said module engages said supports. If the module is not planar, but has a convex surface facing the interposer, the supports serve as stop members spaced apart and extending at right angles to the planar surface of the interposer, and along two edges thereof, into contact with the module. These stops serve to limit rocking motion caused by the convex curvature of the module relative to the planar surface of the interposer. The interposer may also include two contiguous edges containing edge restraints positioned to align the contact sites of the module with the contact pads on the PCB. Springs interconnect the other two edges of the interposer to the module. The restraints serve to limit sliding along the planar surfaces of the module and the interposer.
The invention also relates to an interconnection between one or more contact pads on a surface of a printed circuit board and the corresponding contact sites on a surface of an electrical module. The interposer is positioned between the printed circuit board and the module and includes a compressible, electrically conductive contact for each pad and site. The interposer further includes a plurality of stop members and/or restraints projecting therefrom to limit the motion between the module, the interposer and the printed circuit board caused by the clamping force applied during the assembly process. If the bottom surface of the module is convex, the interposer includes two spacedly positioned stop members projecting therefrom toward said module such that the edges of the module engage only said stop members and the spring elements. The two stop members extend at right angles to the planar surface of the interposer and limit the rocking movement of the module that may occur due to shock and vibration. Means for controlling the relative sliding motion between the module and the interposer comprise two contiguous edges of the interposer containing edge restraints positioned to align the contact sites of the module with the contact pads of the PCB. This also serves to reduce the available surface area of the interposer against which the module can slide. The springs interconnect the other two edges of the interposer to the module.
The invention also relates to a sub-assembly and its method of assembly comprising a rigid electronic module having a generally planar surface and an interposer having a generally planar surface against which the module is clamped. The module can be ceramic, a dielectric, plastic or other rigid material. Means are provided to limit the relative movement of the module with respect to the interposer when the sub-assembly is subject to shock and/or vibration. The limiting means serves to limit relative sliding movement along the x and y axis parallel to the planar surfaces, and comprises restraints along two contiguous sides of the interposer. The limiting means can also limit the relative movement along the z-axis orthogonal to the planar surfaces and comprise at least two stops that restrict the rocking movement of the module with respect to the interposer caused by a lack of planarity of the bottom surface of the module.